The present invention relates to a data transmission system and, in particular, it relates to a digital data transmission system constituted throughpoint-multipoint connection.
A digital data transmission system adopting point-multipoint connection conforming to the TTC (Telecommunication Technology Committee) Standard JT-I430 (reference: TTC standard ISDN User-Network Interface Layer 1 Specifications Standard No. JT-I430 Telecommunication Technology Committee Associated Corporation) is employed in transmission of data such as voice that require realtime quality among a plurality of devices under normal circumstances.
Through the point-multipoint connection in an ISDN user-network interface, a plurality of terminals can be connected through connectors to a main wiring connected to a DSU (digital service unit) to afford advantages such as easy wiring, efficient wiring and easy terminal connection.
Now, an analog data transmission system normally employs a 2-wire metallic transmission line, whereas a digital data transmission system in the prior art adopting the point-multipoint connection employs a 4-wire metallic transmission line comprising two wires each for transmission and reception. Since the number of wires in the transmission line is doubled, the cost of the transmission line is higher. In addition, it is necessary to provide pulse transformers for data transmission and data reception in the individual terminals connected to the main wiring cable, which makes it difficult to achieve a high degree of compactness for the terminals and ultimately results in an increase in the scale of the data transmission system itself.
Furthermore, when a 2-wire metallic transmission line is already wired in a normal residential building, it can be converted to a digital data transmission system adopting the point-multipoint connection either by adding two new wires or by discarding the existing two wires and installing a 4-wire transmission line. However, it is not easy to modify a transmission line regardless of which method is employed, which has been one of the negative factors that deters conversion to digital data transmission systems.
Accordingly, an object of the present invention is to provide a data transmission system constituted through a point-multipoint connection employing a 2-wire metallic transmission line, which achieves efficient data transmission without individual sets of data colliding with each other in data transmission between a master apparatus and a plurality of slave apparatuses.
In order to achieve the object described above, a data transmission system constituted by achieving point-multipoint connection for a master apparatus and a plurality of slave apparatuses is provided. The data transmission system is characterized in that the master apparatus transmits data to the plurality of slave apparatuses during one time block and the individual slave apparatuses transmit data to the master apparatus during another time block. Since this structure allows the data to be transmitted from the master apparatus to the individual slave apparatuses and the data to be transmitted from the slave apparatuses to the master apparatus to be transmitted on the transmission line during different time blocks, data do not collide even if the transmission line is constituted of, for instance, two wires.
The one time block may include a first time block and a second time block. In that case, the master apparatus may transmit a slave apparatus control signal for controlling one or two or more slave apparatuses during the first time block and transmit an information signal for providing specific information to one or two or more slave apparatuses during the second time block.
Apparatus ID numbers may be allotted to the individual slave apparatuses so that the apparatus ID numbers are included in the slave apparatus control signal transmitted by the master apparatus. By adopting this structure, the master apparatus can select one or two or more slave apparatuses among the plurality of slave apparatuses with ease. For instance, by allowing data to be transmitted only to the selected slave apparatuses, data collision in the 2-wire transmission line is prevented to achieve efficient data transmission.
Time slots corresponding to the individual slave apparatuses may be set within the first time block. This will make it possible to transmit slave apparatus control signals for the individual slave apparatuses from the master apparatus without them colliding with each other in a 2-wire transmission line.
The other time block may include a third time block and a fourth time block. In that case, one or two or more slave apparatuses may transmit a master apparatus control signal for controlling the master apparatus during the third time block and transmit an information signal for providing the master apparatus with specific information during the fourth time block.
By setting time slots corresponding to the individual slave apparatuses in the third time block, it becomes possible to transmit master apparatus control signals from the individual slave apparatuses to the master apparatus without them colliding with each other in, for instance, a 2-wire transmission line.
A data detection circuit may be provided at each slave apparatus for monitoring transmission line data that are being transmitted through a transmission line connecting the master apparatus and the individual slave apparatuses, since this will enable the individual slave apparatuses to transmit data in correspondence to the presence/absence of transmission data in the transmission line.
In addition, the data detection circuits provided at the individual slave apparatuses each count the number of continuous codes indicating a specific logic level in the monitored transmission line data and each prompts a data transmission circuit provided at the corresponding slave apparatus to start transmission of transmission data when the number of continuous codes has reached the count value set for each data detection circuit. For instance, in a data transmission implemented through the HDLC (High Level Data Link Control) procedures, under normal circumstances, data xe2x80x9c1xe2x80x9d are not continuous in a succession of 7 sets or more in the transmission line, and if the data xe2x80x9c1xe2x80x9d are continuous, it means that neither the master apparatus nor any of the plurality of slave apparatuses is engaged in data transmission. Thus, by setting the count value as necessary in conformance to a communication protocol, the individual slave apparatuses are able to detect the presence/absence of data in the data transmission line and to transmit transmission data without resulting in the transmission data colliding with data transmitted by the master apparatus or by another slave apparatus.
By increasing the count value when the transmission of the transmission data transmitted by an individual slave apparatus is completed and reducing the count value when the transmission of the transmission data transmitted by the slave apparatus has been stopped, a slave apparatus having completed a data transmission will give another slave apparatus an opportunity to perform the next data transmission. Thus, all the slave apparatuses will have equal opportunity for data transmission.
The data detection circuits provided at the individual slave apparatuses each compare the transmission data transmitted by the corresponding slave apparatus and the transmission line data and if their codes do not match, issue instructions to the data transmission circuit provided inside the slave apparatus to stop the transmission of the transmission data. Thus, even when a plurality of slave apparatuses have started data transmission at the same time, data collision in the transmission line is prevented. In addition, by assigning data that correspond to the apparatus ID numbers of the individual slave apparatuses to the leading bit to a specific bit of the data transmitted by the slave apparatus, a given slave apparatus is selected at an earlier stage. As a result, the selected slave apparatus can transmit a specific information signal within a shorter period of time, the other slave apparatuses are prevented from being engaged in wasteful data transmission and, consequently, a more efficient data transmission is realized for the entire data transmission system.